1. Field of the Invention:
The present invention relates to electrochemical machining method and apparatus using ultrapure water, and more particularly to electrochemical machining method and apparatus which can perform machining for removal, formation of an oxide film, or film formation of a workpiece, such as a semiconductor material or a metallic material, with the use of only ultrapure water as an electrolytic solution.
2. Description of the Related Art:
In recent years, the progress of technology has developed various new materials one after another. However, the machining technology useful for these new materials has not been established yet, and the development of the machining technology has followed the development of new materials.
Further, components in various types of equipments have become finer and have required higher accuracy. As submicronic manufacturing technology has commonly been used, the properties of materials are largely influenced by the machining method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform machining without deteriorating the properties of the materials.
Some machining methods, such as chemical polishing, electrochemical machining, and electropolishing, have been developed in order to solve this problem. In contrast with the conventional physical machining, these methods perform machining for removal or the like by chemical elution. Therefore, in these methods, alteration of a machined layer due to plastic deformation and defects such as dislocation are not occurred, so that machining can be performed without deteriorating the properties of the materials.
Further, attention has been directed to a machining method utilizing interatomic chemical interaction. This method utilizes fine particles, radicals having highly chemical reactivity, and the like. According to this machining method, machining for removal or the like is performed by a chemical reaction with a workpiece on an atomic level. Therefore, machining can be controlled on an atomic level. This type of machining method includes elastic emission machining (EEM) and plasma chemical vaporization machining (CVM), which have been developed by the inventors. The EEM utilizes a chemical reaction between fine particles and a workpiece, and can realize machining on an atomic level without deteriorating the properties of materials. On the other hand, the plasma CVM utilizes a radical reaction between a workpiece and radicals produced in plasma at atmospheric pressure, and can realize machining on an atomic level.
In the aforementioned electrochemical machining and electropolishing, it is considered that machining proceeds through electrochemical interaction between a workpiece atom and an electrolyte ion in an electrolytic solution (an aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH, or the like). The contamination of the workpiece with the electrolytic solution is unavoidable as long as the electrolytic solution is used.
Accordingly, the inventors have considered that, in neutral and alkaline electrolytic solutions, machining is related to hydroxide ions (OH−). This consideration has led to such an idea that machining can be performed even with water containing a small number of hydroxide ions. The inventors have experimentally confirmed the feasibility of this technology, and have proposed a method of increasing the ionic product in ultrapure water, in which a small number of unavoidable impurities is contained, as disclosed in Japanese laid-open Patent Publication No. 10-58236.
According to the above method, a workpiece is immersed in ultrapure water having an increased concentration of the hydroxide ion, and machining for removal or oxide film formation is performed through a chemical elution or an oxidation reaction with the hydroxide ions. Further, the inventors have also proposed the utilization of an electrochemical reaction on the surface of a solid having an ion exchange function or catalytic function to increase the number of the hydroxide ion. As a result, there has been developed a novel machining method which can realize machining in a clean manner without leaving impurities on the machined surface through the utilization of hydroxide ions in ultrapure water. This machining method has been expected to be used in wide applications including the field related with semiconductor manufacturing. Thus, the inventors have proposed a machining method using ultrapure water as an electrolytic solution, which is a low-damage machining method utilizing a chemical reaction, and a clean and low-environmental load machining method.